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What is the actual air speed over and under a wing due to Bernoulli's Principle?
Bernoulli's Principle states that as a fluid speed increases, its pressure decreases, and vice versa.  Air flowing over the wing of an aircraft flows faster than its neighboring air flowing under the wing.
But how much faster?  Using the cruise conditions listed for a Boeing 747-400 – airspeed Mach 0.85 (567 mph, 493 knots, 912 km/h) flying at 35,000 ft – what are the corresponding maximum and minimum speeds of the airflow over the wing?
For the purposes of the calculation, assume standard atmospheric conditions for an EAS of 273 knots.
In general:

How much does this depend on the shape of the airfoil (or: does this translate to general aviation as well)?
Does changing the angle of attack (without stalling) affect the airspeed/pressure differential?  If so, by how much?
Typically, where on the wing (inboard to outboard) is the most Bernoulli-derived lift generated (i.e. due to airspeed/pressure differential)?  Or does this vary greatly with wing design?
Where on the airfoil (leading edge to trailing edge) is the region of highest and lowest pressures (and thus, the slowest- and fastest-moving air)?

This question is persisting a long held myth. Bernoulli can predict pressure differential, but lift is simple Newtonian reaction to pushing more air down then up.

If you know the pressure coefficient of the flow, the rest is easy. The equation for the pressure coefficient $c_p$ is:
$$c_p = \frac{p - p_{\infty}}{q_{\infty}} = 1 - \left(\frac{v}{v_{\infty}}\right)^2$$
$q_{\infty}$ is the dynamic pressure and contains air density $\rho$ and the flight speed $v_{\infty}$:
$$q_{\infty} = \frac{\rho}{2}\cdot v_{\infty}^2$$
The equation for the local speed, relative to flight speed, is:$$\frac{v}{v_{\infty}} = \sqrt{1 - \frac{p - p_{\infty}}{q_{\infty}}} = \sqrt{1 - c_p}$$
Now you need to know the pressure coefficient. At Mach 0.85 the local Mach number of the 747 wing, which has a wing sweep of 37.5°, is 0.674. Since I do not have the airfoil of the 747-400, I used one from the same family (BACJ), found here. If you are interested, many more can be found in Michael Selig's immense airfoil database.

The plot is from a TSFOIL calculation of the 2D flow around BACJ at Mach 0.8, so it needs to be taken with a grain of salt. Note that it shows a stagnation point, and, therefore, Mach = 0 at the leading edge. The leading edge of the 747 wing is swept by 45°, so it will only slow the orthogonal speed component to zero. Consequently, the 747 wing has a stagnation line where the lowest speed will still be Mach 0.6.
On the suction side, the airfoil reaches Mach = 1.2 quickly and even Mach 1.3 near the trailing edge. Modern, so called supercritical airfoils can tolerate mildly supersonic speeds on their suction side, and the Boeing 747-400 makes use of them. However, since the wing sweep reduces Mach effects, the top speed will be Mach 1.2 or slightly lower, and the TSFOIL calculation shown here is not exactly correct for flight at Mach 0.85, but gives a general idea what happens on the wing.
To answer the many questions you posted directly:

The shape of the airfoil is immensely important. Thicker airfoils need to displace more air, and higher flow speeds are created on both sides. Camber will increase the pressure (and speed) difference between both sides.
Changing the angle of attack will add a roughly triangular difference, which is highest at the leading edge, to the difference between both sides. Lower side pressure is increased slightly, and upper side suction goes up by a lot at the nose.
Good wing designs have a triangular lift distribution over wing span, with the highest lift at the root. Since the displacement effect of the fuselage is added to the flow effects, the highest speeds will be encountered at the wing root.
Just look at the pretty pictures ...

excellent detail, especially the graphs.  the 737 is also a good candidate for discussion – my interest is in commercial aviation in general, as this sector is most interested in range and efficiency.  i just had to pick a baseline model so i didn't get all sorts of answers, but i imagine wing shape is fairly consistent in this aviation sector.

if i read the graphs right, there is pretty consistent pressure differential along the entire chord.  and, the underside of the wing also sees a decrease in pressure up until about 2/3 down the chord, and conversely increased speed in the same region.  overall lift differential can be computed by subtracting integrals.

do the graphs depict mach 0.85 or 0.675?  looks like the latter is the baseline on both.  if so, it is interesting to find that even at that reduced speed, airflow hits mach 1.  (even more interesting where it reaches that speed.)  if airspeed was increased to 0.85, does the overwing speed increase accordingly?  you mentioned supercritical airfoils able to tolerate mildly supersonic airflow.  that implies to me that the sound barrier acts as "soft" ceiling, and airflow rises to meet it, regardless of baseline airspeed.  so it would be interesting to see what mach 0.5 looks like as well.

@erich: I redid the post with another airfoil, which is much closer to the real thing used on the 747, but unfortunately the author of the paper from where I copied it has not done a good job of picking the correct speed. They used Mach 0.8, which is between the flight Mach number of 0.85 and the Mach number of the unswept wing of 0.675. Unfortunately, I have no coordinates and can, therefore, not improve the plot. At least not now.

Wishing I had my copy of Basic Aerodynamics handy to provide some more detailed references and numerical examples (may come back later to add more detail), but here goes the highly abbreviated version.

Airfoil shape is absolutely critical in determing the distribution of lift (and therefore air pressure differential between the two sides of the wing) and this holds true for literally anything that makes use of them. The exact specifics as to how much this varies differ from one airfoil to the other, but you should be able to just look up the airfoils you're interested in and compare their coefficients of lift, one of the variables in the generation of aerodynamic force (i.e. horizontal lift and induced drag), at various alphas (a.k.a. angle of attack, angle of incidence).
The coefficient of lift is indeed determined by the angle of attack. The greater the angle of attack, the greater the coefficient of lift, right up until the point where airflow separates from the wing and you enter a stall. The end result is that, for a given airfoil and constant airspeed, the higher the angle of attack, the more lift your wing will generate, and the greater the air pressure differential.
A very interesting question and the answer is, it depends. For a constant camber airfoil (think your basic Hershey-bar style wing), you should be able to get more lift being generated closer to the wing roots. This happens due to the fact the airflow tends to "slip" around the wing edges due to the fact that we do not have infinite span wings and the higher pressure air underneath the wing tries to fill the lower-pressure areas on top and has an outlet to do so where the wing ends. The end result of this phenomenon is known as a wingtip vortice and one of the effects is that the aerodynamic force vector usually has a somewhat different direction at the wing edges vs. the tip, usually leading to less vertical lift being generated their as opposed to the wing root. Other things to keep in mind, many modern manufacturers will use variable-camber designs today to build wings that stall at the wing root first in order to ensure aileron control as far into the stall as possible. Recommended further reading on this topic: wingtip vortices, wake turbulence, winglets, Spitfire wing
For most subsonic airfoils, the center of pressure will occur quite close to the leading edge. Again, this depends on the wing design itself, which is dependent on aircraft role and requirements. Aircraft designed to fly slowly will typically have relatively "fat" wings with a pronounced camber close to the leading edge. Aircraft design to fly faster will seek to reduce frontal area and will have their thickest camber set farther back. Supersonic airfoils generally have a vaguely diamond-shaped profile owing to the need to keep the leading edge very sharp and the frontal area minimized.

If I might suggest, if this is something you're curious about, look into picking up an introductory text on aerodynamics. The topic is truly fascinating and the answers to your questions can run very deep indeed.
In the meantime, hope this helps.
Follow-up
@erich @FreeMan I've been doing some thinking about the 747 question and I think it's worth delving into some of the underpinnings to clarify a few things.
Disclaimer: My expertise here is as a pilot rather than an aero engineer (big, big difference in the depth of study of aerodynamics), so if any mistakes follow, that's why.
One thing to remember about an aircraft in flight is that it's not so much air "flowing" over the wing, as the aircraft "flowing" through the air. We often illustrate air moving over the wing because this is how it works in wind tunnels and, as an added benefit, it simplifies things when introducing this topic to new students. We often use the term "Relative Wind" to refer to this phenomena.
As such, it is important to remember that when the aircraft moves through the air, it displaces air and pushes it above and below the wing. To further simplify our discussion, let us assume a positively-cambered wing at 0 alpha. What will happen in this situation is that the surface area on the top of the wing, being greater than the surface area below the wing, will have fewer air particles per unit of area leading to lower pressure on the top of the wing than the bottom, as air density is relatively constant around the aircraft. This pressure differential results in a force, whose vertical component counteracts the aircraft's weight, resulting in level flight. Extrapolate from here for climbs, descents, banking turns, etc.
Digression: you may point out that if this is the case, how come airplanes don't just sort of float away when they're on the ground, seeing how the wing area differential still holds. The intuitive reason for that is that unless the aircraft is moving and "disturbing" the air around it, air particles will be free to float around the wing and fill any low pressure areas until the pressures equalize.
Going back to our 744, what you would really be interested then is to actually calculate pressure differentials and, working from there, calculate "speed" differentials (keeping in mind that you would actually be calculating more of an "average speed", as different parts of the wing will generate different amounts of force). Someone else will have to do that math however, as I can't seem to find the necessary technical data. Also, in case you're wondering, yes, the fuselage also generates a certain amount of lift, while the tailplane horizontal stabilizers generate a aerodynamic force downwards (think "negative lift"). Have fun digging :)

great start! just a minor course correction here: i'm interested only in the bernoulli-related effects of changing aoa in question 2, not just increased cL.

Excellent answer. You missed the first, unnumbered question, though - how much faster does the air flow over the top of the wing than underneath, specifically for a 747-400 at Mach 0.85.

@erich - one way that you can think of with respect to changing the aoa is of a pilot-driven change in the wing's camber. Everything flows from there i.e. "thicker" camber => greater pressure differential (i.a.w. Bernoulli) => greater cL => etc.

Experiments with smoke have shown that when a mass of air is separated in two parts by the leading edge, the part on the bottom of the wing stays in contact with the wing longer than the part on top, and after the airplane passes the two parts remain separated by a considerable distance. The speed difference over the wing's surface is much larger than the distances along that surface would indicate. This also helps explain why the aircraft doesn't magically float away while sitting on the ground.


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Why EU/US airline checked baggage allowance limited at 32kg?
For most full services on EU/US airlines, the permitted weight of each of the checked baggage allowance is 32kg. It is similar for the routes operated by non-EU/US airlines, even though their overall checked baggage allowance is more than that. What are the reasons behind that?

Presumably it's as small as airlines can get away with. With my cynical hat on, this will increase the number of excess baggage surcharges...

For the specific value, 23 kg is 50 lbs., so it's probably something that started in the US.

I don't quite understand your question, perhaps you could try to clarify it? Most airlines allow you to push the weight up to 32kg/70lbs as an absolute limit (possibly with extra charges). Heavy bags slow operations. 5 seconds extra on 100 bags is 8 minutes extra of work. They also cost expensive fuel: for instance 5kg extra among 150 passengers on an A320 would weight 750kg, in relation to an empty weight of 61,000kg.

@MikeFoxtrot  Thnak for your comment. It is 32kg not 23 kg. But i know "Heavy bags slow operations", what i ask is why not 31/33kg?

I guess it's just a simple rounded imperial measurement unit of 70lbs that was chosen in the past, perhaps by some organization, and that has stuck and been accepted. IATA also follows it.

@Him Same reason why they'd use 23kg for luggage that doesn't require an extra charge: the real number is 70 lbs, probably originating in the US (or the UK pre-metric). People elsewhere adopted the same number, but they didn't use pounds, so they converted it to kg (where it rounds to 32 kg). Similarly, a football goal is 7.32m wide and 2.44m high -- it started out with different units where the numbers make sense (8 yds by 8 ft), and was then converted to metric. This is often the case when you find odd specification: it's converted from a measurement system where it was a nice round number.

@cpast You misunderstood my question. I am asking why it is 32kg/70lbs but not 71lbs/69lbs, not why it is using metric system but not imperial units.

@Him Because 70 lbs. is a nice round number. They did not set it based on detailed analysis; they decided 70 lbs. was about right (for whatever reason), and didn't go more precise than to the nearest ten pounds because there's really not much point.

It could be to have universal requirements. On jets, this probably isn't an issue, but think of all the hard working regional jets and turboprops.

So... exactly what do you need to carry in your luggage that weighs more than 32kg?

To add to Antzi's answer it is most likely about worker safety. Your question is where do the 50lbs/23kg and 70lbs/32kg numbers come from. The current answer for US based airports would be the recommendations of the Occupational Safety and Health Administration (OSHA). Here is a link to their current lifting standards. There is a neat worksheet down at the bottom for calculating the weight a worker should lift based on the types of movements they have to perform, distances traveled, angles bent. It is a system based on multipliers whose base limit on a normal lift is 51lbs/23.4kg. Then 70lbs/32kg would amount to the multipliers all working out to 1.4 which is again a nice round number.
I would also speculate that at some points these numbers made it into union contracts for ground workers but I have no actual information on that.
What I couldn't find were the OSHA regulations that were in force when IATA first enshrined 23 and 32kg into their rules. It is perhaps a bit disappointing that the current OSHA regulations to not have a "max lift" number but their interest is in reducing injury and a single very heavy lift is apparently less damaging than repetitive above average lifts.

Bolstering the "worker safety" aspect is the US Postal Service package weight limit (also 70lbs / 35 kilos, and imposed for the safety of mail carriers who have to haul the package around). For their part OSHA doesn't have a specific regulatory limit as far as I'm aware, but if you poke around they start making noise about lifting anything heavier than 50lbs (22.5 kilos).

(+1) All multipliers in the equation are below 1, though. On the earlier version see https://www.longdom.org/open-access/the-niosh-lifting-equation-for-manual-lifting-and-its-applications-2165-7556-1000159.pdf

Because people have to handle your luggage by hand, which is quite a hard job.
Heavier luggages would pose a health threat for them, putting even more stress on their back.
I once flew with a heavier (>32kg) luggage on a Seoul-Taipei flight and the ground staff was not pleased at the checking counter...

Any other reasons?

I think it is a valid reason.

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Why do aircraft manufacturers produce families of aircraft?
Let's take the case of the Airbus A320 family of aircraft. Airbus manufactures A318, A319, A320 and A321 planes, which all belong to the same family.
What is the need to manufacture multiple aircraft from the same family having different ranges and capacity, when one could just invest in making just two or three different planes from three different families providing a wide range of range (distance) and capacity?

Note, that distinction between A318-A321 is comparable to distinction between B737-100-B737-900. A320 is the only type where the main number is varied, but almost every type has multiple variants.

It was just an example

I understand. I am just noting that that particular example is special in the naming, though not in principle.

Saying that a company could "just" make several completely different products instead of making variants of a single product is exactly backwards. The use of "just" implies that it would be the easier alternative but it seems completely obvious that it isn't. I'm not sure why you think it would be easier to design multiple independent products than design variants of a single product.

See this answer or this. Make sure to read the comments, too.

There are several advantages, here are a few:

Design costs are lower for a family of aircraft; major parts of the aircraft are the same in the family.
A family of aircraft share the same type certificate. This means it is cheaper for the manufacturer to certify a family of aircraft.
Within a family of aircraft, pilots can operate all aircraft with no
or very little additional training required. This saves the airline a
lot of money.
Maintenance on a family of aircraft is cheaper since the majority of
parts are common to all family members. This means having less spare
parts in stock and less training for maintenance personnel.
For an airline there is often a benefit in having some capacity
diversity in a fleet. A new route often starts at low capacity. When
the volume of passengers rises replacing the flight by a  larger
aircraft from the same family is not disruptive to crew schedules.

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Why is the initial short-field climb below Vₓ in a Cessna 172?
Short-field takeoff procedure for a Cessna 172R is

Wing Flaps – 10°.
Brakes – APPLY.
Throttle – FULL OPEN.
Mixture – RICH …
Brakes – RELEASE.
Elevator Control – SLIGHTLY TAIL LOW.
Climb Speed – 56 KIAS (until all obstacles are cleared then $V_y$).
Wing Flaps – RETRACT slowly after reaching 60 KIAS.

However, best angle-of-climb or $V_x$ at is 62-67 KIAS depending on elevation. Why is the first stage of the climb slower than $V_x$, and if the slower speed gives better performance, then why isn’t that the published figure?

$V_X$ published in the manual is $V_X$ for flaps up. $V_{X_{F10}}$ will be slower than $V_{X_{UP}}$. Most likely, the speed in the procedure is, or is very close to $V_{X_{F10}}$.
$V_X$ (speed for angle of best climb) occurs where the excess thrust ($T$ - $D$) is the highest. Refer to the graph below:

$D$ is Induced drag ($D_I$, varies with $\frac{1}{v^2}$) + Parasite drag ($D_P$, varies with $v^2$). By extending the flaps, you are increasing only $D_P$ and the graph describing $D$ ($D_{tot}$ in the graph) moves upwards to the left. This means that the point for $V_{MD}$ moves to the left. Since $V_X$ is found at max $T - D$ it varies with thrust too (which will vary differently, depending on the type of propulsion system), and is not automatically found at $V_{MD}$, but the two speeds generally move together in the same direction.
There's also another, albeit marignal, effect (that i did not put in the graph) to take into consideration for the first 10-20 ft, and that's the ground effect. In ground effect, the aircrafts $D_I$ will be lower, further moving the $D$ graph to the left, and downwards, resulting in a lower $V_{MD}$, and therefore generally also lower $V_X$

The published Vx is in the clean condition (no flaps): 62.
With the flaps down, for a short-field takeoff, Vx is 56.
This is the same reason why the stall speed (bottom of the white arc) is lower with flaps than without (bottom of the green arc)

Small GA POHs can be fairly sloppy (and your checklist seems even looser, what is your Vr? Why are you going rich after full throttle?).
POH's often will fail to properly define things. For example is a short field(aka maximum performance) takeoff the shortest ground roll, or the shortest distance to clear a 50ft obstacle? That difference needs to be known before you can get a meaningful answer to your question.
Vx (62) is best steady-state climb angle at standard sea level, like if you have a large hill to climb over, it increases and approaches best glide speed at your absolute ceiling.
However, a takeoff is not a steady state condition, it is a dynamic combination of factors. The best 50ft obstacle distance is the best angle measured from the beggining of the runway(brake release point), not the angle measured from the end of rotation. Depending on density altitude, gross weight, and especially headwind; 56kIAS with flaps-10 may shorten ground roll and start the climb earlier more than compensating for a lower steady-state angle of climb.
This isn't even adding the effects of a zoom climb. (Although the POH procedure would be based on such a maneuver.) Increase speed on the ground or in ground effect, then continue rotation to a higher pitch and convert speed into altitude pitching down slightly as you get to Vx. Actual advantage will vary based on many factors.


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How is a cruise missile's range extended?
I see that some countries periodically test their cruise missiles, saying that they have increased the range. For example, Pakistan doubled the range of the Babur cruise missile and designated it as Babur 1B.
My question is, what kind of modification is required to extend the range of an existing cruise missile?
Do they re-engineer the engine? Do they extend the fuel capacity? Do they change the type of fuel?

It's probably the same as extending an airplane's range, which is a pretty general answer.

Better fuel, better electronic (and sensors) which can optimally burn fuel. Better simulation of engine to improve aerodynamics and internal fuel flow. Lighter materials (on some parts). More fuel. Etc. but it is military stuff, so we know very few (and sometime misleading facts). Space Stack Exchange may have better opinions, because technologies may be the same as space launchers.

It doesn’t necessarily need to be engine/fuel related. It could also be an improved guidance system that allows the required precision over a longer distance.

This is also trending well into politics - range extension can simply mean 'we lied about the before range and now choose to tell a different lie', or did things like replacing the warhead with a fuel tank or chose ideal environmental conditions for the flight.

One obvious way is to replace the engine with a more fuel efficient one.
Swapping out parts made out of metal with carbon fibre parts is also an option, reducing weight will increase range. Reducing the warhead weight does the same thing, obviously, but also reduces combat effectiveness.
Air launching instead of launching from the ground can also increase range as the climb to cruise altitude from the ground through thick air costs a lot of fuel (but I guess that's not what you're looking for as a way to increase range), or alternatively replace the launch booster rocket with one that has a longer burn time.

As pointed out by the comments, I think extending the fuel capacity, for example by ellongating the body of the missile is also a very obvious solution.

@U_flow yes and no. It means changing the size and shape of the missile body, which may or may not be an option depending on launch mechanism. E.g. a submarine launched Tomahawk is limited by the size of a standard 21" torpedo tube.

good point, it depends on the constraints for example because of the launch mode. However the elongation is simple and largely preserves the Cog and aerodynamic properties, as well as it is simple from a production perspective while yielding the greatest range boost. I speculate that this is the method was used in this case, but I have no proof.

@U_flow lengthening the missile body tends to change the aerodynamic properties, especially center of gravity, which may necessitate other changes (if you're lucky just to the avionics software, if not to the wings and other surfaces).

given other options I still argue that such a modification might be the most effective option

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Why is the A380 a double-decker rather than a longer airplane?
It seems like making the A380 a double-decker increases the cross-sectional area of the fuselage, thereby increasing drag on the airplane. Wouldn't it be more efficient to fit so many seats in a longer airplane rather than a wider airplane?
So, is the A380 a double-decker because there is a limit to how long an airplane can be, would it be impractical to make a longer airplane, am I wrong about the cross-sectional area/length trade off when it comes to drag, or is there some other reason?

That's right, having a longer plane would reduce drag by lowering the cross section (altho there is also drag linked to the surface area of the plane).
There is a few reasons why you don't want to do it:

Cylinders don't scale this way:

Take a cylinder with a radius of 1 and a height of 10.
Volume is ≈31.41
Double the height, volume is ≈62.83
Double the radius, volume is ≈125.66
Increasing the length scale linearly and increasing the volume scales at π * n * n

You will need a stronger structure to support the additional length and surface area of your pressure vessel, oscillations during turbulences, takeoff rotation, ...
All your cables, hydraulics, fuselage, will have to be stretched
Passengers will take forever to reach the back of the plane when boarding
Maneuvering in the airport will be impossible, you will need new airports  (credit apple maps)

TODO: add area comparison ( a good proxy for weight?),

Skin friction drag is not a marginal factor.

Though WRT structural strength, I can't help but wonder about lengthening the aircraft by adding another set of wings...

@jamesqf Stop playing KSP :) The main issue will be a disturbed airflow for the back wings

Would not mind somebody stepping up and designing a modern biplane.  Dunne was on the right track with swept wings and closing the ends by placing vertical stabilizers there.  Some designers want to bring the second wing back to the tail, but this may be less stable than good old Tiger Moth.  A biplane, with GE fan jets, could carry a lot of payload, or go higher.

Your assumption that a longer fuselage has lower drag is incorrect.
Optimal pressure drag (form or diameter driven) vs skin friction drag (surface area driven) can be determined using fineness ratio. From Roskam and considering subsonic aerodynamics only, the optimal ratio is 6:1. Including other factors such as ease of use or manufacture, the ratio can go higher.

The A380 fuselage at 7.15m wide and 8.4m tall is 7.77m average diameter. It is 72.7m long so even as a double decker it is more than 9:1, higher than optimal. The curve is very flat beyond 4:1, so the length of the A380 is driven by considerations other than fuselage aerodynamics which mildly argue for an even fatter fuselage.

When designing the A380 Airbus made the decision that it would have to be able to use existing airport infrastructure, so the footprint could not be much larger than that of a Boeing 747. If they made it longer or gave it a wider wingspan airports would have to be redesigned to serve it, for instance gates would have to be spaced farther apart, taxiways would have to be changed to give clearance and accommodate a wider turning radius, These costs would have been prohibitive and the airplane would not have found customers.
Given the footprint constraints a double deck concept was the only workable solution for the number of passengers they wanted to fit.

Even with staying in the 747 configuration, airport gates had to be redesigned to accommodate the A380. At many airports closes the adjacent gates when boarding because of wingspan. Nobody would fly it if they had to reengineer taxiways and runways. It's an amazing plane, but issues like this have hampered sales.


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Is it possible to avoid vomiting while practicing stall?
I found that even after 30 flying hours in a Cessna, my body still cannot tolerate practising stall, thus throwing up and dizziness follow up after the sorties. Is there any way I could prevent that?

It's that drop on stall that messes with the senses.  One might be careful with eating before flying.  Also, practicing stalls and recoveries do not necessarily involve a full blown stall, then having the nose hurtling towards earth, followed by the pull-out (putting >1 G on the stomach).  With the Cessna 172, relaxing the elevator as soon as the nose drops is a much gentler recovery.  (On buffet onset is even better!).  Provided CG is correct, recovery can be that easy.  Seek advice from a professional instructor.  You may be making this more difficult than it needs to be.

This seems like an opinion-based question.  It is obviously possible since I never had that feeling ever when practicing stalls.  But whether any one person having felt so can overcome that feeling can only be answered on a case-by-case basis.

Get travel chewing gum. Helped me a lot when I was routinely riding in ambulances sitting with my back in the driving direction and noting down patient data.

@Michael while this works, those have in the patient information that you shouldn't fly with them.

I have found tensing up my stomach before some manoever helps.

@Jim The technique used might vary, but I'm pretty certain one can objectively state that it is possible,  unless vomiting and dizziness is sufficient to immediately preclude the possibility of getting your pilot's license.

@chepner - That’s a pretty big caveat and one under which my statement was not made.

There is a drug called meclizine.  https://www.accessdata.fda.gov/drugsatfda_docs/label/2012/010721s058lbl.pdf

Practice. That sounds dumb, but is true. The more you practice this, the more you get accustomed to this motion.
Some scientific backup: The most motion sickness inducing motion is a vertical oscillation of 0.2 Hz. The nearer you are at this frequency, the more motion sick you will get. Additionally, around 5.7% of military student pilots experience motion sickness during their first training flight. This figure drops to around 1.1% after their 10th training flight (though this might be biased, as military student pilots have an inherent interest not to be motion sick).
(Source: Page 2, Motion Sickness, by K.E. Money from 1970)
More on this topic in this question.

Are there statistics as to how many USAF pilots who have a first training flight also have a 20th? (I'm wondering if there may also be some survivorship bias in that stat. But I have no idea what if any weed-out process the training has.)

@yshavit I located the exact source, I updated my answer with the source from which I pulled the numbers. It furthermore states: "training is discontinued because of motion sickness in 5 % of student navigators [...] and in
approximately 1 % of student pilots [...]. In one smaller sample of student
pilots, 6.7 % ceased training because of motion sickness [...]. " Please note that this paper is from 1970

But again, bear in mind, that this is military training. Part of the (at least perceived) job description is to look tough. Therefore 1% of the pilots admitted to have aborted the flight training because of this.
Additionally, I misquoted. The numbers above are given for the 10th training flight. I updated the text above.

Great updates, thanks!

It goes away after enough time. It's the same thing as acquiring "sea legs", getting over sea sickness on board a ship.
What can help is go ride on a park swing which, being repeated 0G events, can create sea/air sickness in a lot of people. This can help desensitize you to the motion sense/visual contradictions that generate the nausea and speed up the adaptation process.
If I spend enough time on a park swing, I'll still get a bit nauseous, and I've been flying since the mid 70s. And if the average pilot spent an hour doing aerobatics with an expert, most of them will be hurling their lunch before they're done. So don't give up yet.

We should be honest here: Motion sickness does not necessarily subside ever. A considerable percentage of people do not become impervious to it.

You need to find out one way or another, and do what you can to desensitizing yourself.  If he doesn't get airsick in normal flying around in bumpy air, only during extreme maneuvers, that suggests a sensitivity that can be adapted to with exposure.  Giving up prematurely isn't the answer. Some people that go into space puke quite a lot until they get their "space legs".

I'm not suggesting giving up, I'm just stating the facts.

And some of us were born with it. Tire swing just spinning in a circle was my favorite. Being dizzy is fun if it doesn't give you nausea. I just can't go upside down; then it's all over. I grew up across the street from a park. I'd spend hours on the swings; the good ones that were like 20' high. Wrap the two outer ones on the support posts. Now you can do crazy spins on the middle one, at the tops of each arch. Someday I'll get to ride a centrifuge....

If I spend enough time on a park swing, I'll start to feel high. IDK why my inner ear has anything to do with my serotonin uptake, but man, I sure love me some swinging.

@Mazura after about 15 min on one I have to get off.  Never had any problem with airsickness while flying tho.

I don’t want to be discouraging or negative, but some people are just not aeronautically adaptable.
I’m not saying you are or are not, just that while it is possible to lessen the discomfort with repetition, medication, and techniques such as looking at the horizon, extreme sensitivity may prove insurmountable.  At some point besides rendering a normally enjoyable experience unpleasant, debilitating motion sickness (such as throwing up) can hinder performance and become a safety concern.
30 hours isn’t a lot, so keep trying and perhaps you can overcome it, but be realistic in evaluating your chances and reasons why you are choosing to do this.
Good luck!

Well, don’t be too hard. Some of the best pilots in the world suffered from severe air sickness when they began flight training, including Bob Hoover and Chuck Yeager.

For every Hoover and Yeager there are numerous pilots-to-be who never got accustomed to flying.

This has indeed potential to be lot subjective, but my five cents ... (disclaimer: I have no first-hand experience of motion sickness myself, so following is more guessing what could be useful):

Look out (all the time, ignore instruments), try to not lost outside references during whole motion and be constantly aware what is attitude of the airplane by visually tracking surrounding. Experiment with suitable point/direction too look in order to not get lost. Lifting your eyes up and keeping them at horizon while airplane nose drops could be a good starting point, at least if roof won't get in the way.
Be the one who is actively flying and controlling the airplane all the time. Don't get you catch by surprise that "things happening by themself". You can try for example to hold airplane flying low speed at the edge of stall (which is good practice regardless) and then intentionally nudge it into full stall in the moment when you decide to (not waiting for it to happen by chance). Have a plan what exactly should happen then and try actively to fly the airplane so. (It is not so important how precise your flying will be in reality, the important part is that you are not only passively carried by events but (mentally) in control.)

makes some sense. When I drive a car, I have noticed that when I expect car to decelerate dues to brakes but it doesn't (for example being on ice or gravel or in mud), then I feel slightly as if my stomach turns. So physical sensation is very much related to mental anticipation of what's going on.


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Will two counter rotating vortices cancel each other?
Suppose that an aircraft flies with its wing through a tip vortex of another aircraft which flew in the opposite direction. Suppose that the shed wing tip vortex of both aircraft are exactly the same in strength, only rotating in opposite directions?
What will the net effect be? Will the angular momentum be canceled?
I would like to place this in the light of the existence of a root vortex experienced by wind turbines. From Burton's Wind Energy Handbook (3rd edition, p78) :

For example, on a two blade rotor, unlike an aircraft wing, the bound circulations on the two blades shown in Figure 3.27 are opposite in sign and so combine in the idealised case of the blade root being at the rotational axis to shed a straight line vortex along the axis with strength equal to the blade circulation times the number of blades.

With the aircraft example the circulation is opposite in sign as well. So why is the root vortex a summation of the two blades instead of cancellation?

Well, suppose we view aircraft 1 from the back and look to the left wing. The resulting rotational motion will be clockwise. Then, when aircraft 2 approaches, we see it from the front. When we look at its left wing (which is on the right form our perspective) it will be a counter clockwise rotation.

Keep in mind that a vortex is a 3D shape and generally it grows in diameter as the distance between it and the wingtip increases.  So in order to fully cancel, you must have two vortices of identical shape, size, and rotational characteristics EXCEPT that the direction of the rotation is opposite.  Under these very unusual conditions the two would cancel.  Outside of a laboratory environment, I don't believe creating these conditions is possible.

Even in a lab, it'll only happen on the third Tuesday of the month, and when the cameras aren't working.

@jwh20 I do not think those conditions can be met unless both wings are in the same place.  Usually that makes a big explosion and then there are lots of vortices.

@CamilleGoudeseune I'd say the cameras will be permanently not working.

Well, of course it's a simplified problem. Nevertheless, simplified approaches such as lifting lines codes or panel methods have been proven to provide more than satisfactory results in lots of cases. It is in that light that I ask the question. Namely, the citation of Burton points to a vortex line representation of a wind turbine. It is there where I fail to follow the author and why I start a discussion here.

@sophit Looks like you linked my answer instead of yours

@AdityaSharma oops, right, sorry

Have you come up with an idea why the root vortices sum up together instead of cancelling each other?

No I have not. This is the point; if we settle on the aircraft example where they cancel out then I do not see why in the wind turbine case they add up. This is why I started with the aircraft example, which seems to have caused some confusion and no one addressed the root vortex of a wind turbine except that it is a complicated structure. Of course it is, but we're dealing with a conclusion following from a model, I am not asking for the validity of that model.

Obviously having two perfectly identical and opposite vortices generated in exactly the same space is just an ideal mathematical exercise. I suppose it would something like having two billiard balls moving one against the other along the exactly same trajectory and at exactly the same speed... and that multiplied for all the air molecules!
Anyway something similar actually happens also in reality: with contra-rotating propellers. And luckily enough, this interesting report is just right for our purpose. It shows flow measurements behind naval contra-rotating propellers. Measurements are done in three steps: with propellers stopped, with only one propeller rotating and with both propellers contra-rotating. Unfortunately it doesn't deal with isolated propellers since also the stern of the ship is present in the model. So first of all we have to understand how the water flows due to the hull when the propeller are not rotating:

The (black small) arrows in the plot tell us in which direction the water is locally flowing while the background colours tell us how fast it is flowing (blue is slow, red is fast). The three big white arrows "connecting the dots" are my own work and show more or less the path of the water flow. Following them we see that what the water does in this case of not rotating propellers, is basically closing behind the hull: it goes up and then bend to rejoin in the centre of the picture just behind the propeller axis where the ship ends and then the flow just "dies" there. This picture of the hull should help in understanding this movement (C is the plane where the speeds are measured):

Now, what happens when only one propeller rotates?

We see that the propeller causes the water to rotate anticlockwise around it: the same three white lines of before now clearly bend around the propeller. This whole movement of the wake basically resembles the movement of the tip vortex at the wingtip as seen from behind.
Finally, in the next picture both propellers are in movement:

The three white lines are now again like in the first picture: the contra-rotation of the second propeller has more or less restored the symmetry of the water flow, cancelling the anticlockwise dragging of the first propeller. This image resembles indeed the "smiley face" in the first one: obviously here we have more red than blue (i.e. higher speeds) since the propellers are doing their job accelerating the water, but the anticlockwise rotation given by the first propeller is now more or less balanced by the second one.

Will two counter rotating vortices cancel eachother?

Yes, almost completely, even in reality.

Your answer is good, but answers not all my questions. So given that counter rotating vortices of equal strength cancel eachother, why then are the root vortices added to a single line vortex. Clearly, the vortex from the root of each blade rotates in the opposite direction. Hence, if they're shed along the same line, which happens when the blade extends to the center of rotation, why don't they cancel?

Your question is if two contra-rotating tip-vortices cancel each other and the answer is yes (more or less). The vortex structure in a helicopter/wind turbine wake is much more complicated than a wing's tip vortex due to the rotating nature of the entire wake. Note that the two root vortices can never be perfectly aligned due to the hub cutting the blades; plus they have a very weak intensity due to the small lift's change toward the roots.

The actual conclusion stated by your reference is completely different from your answer. "While the tip vortex generated from forward propeller was almost disappeared, the tip vortex and shed vortex generated from aft propeller were observed clearly. Through the axial vorticity distribution on the longitudinal plane along the propeller shaft, it was noticed that the tip vortex of aft propeller became stronger and its contraction increased due to the interaction between the tip vortices of forward and aft propellers."

@AnonymousPhysicist: in my answer I'm dealing with the whole vortex shed from the propeller. The whole vortex shed from the propeller resembles the tip vortex shed from an airplane's wingtip. I stated it in my answer: "These movement basically resembles the tip vortex at the wingtips". Anyway I'll modify this sentence to make it clearer

Now let's assume the two airplanes do not collide but two of their trailing vortices happen to have their core along the same line. Yes, those two vortices will cancel each other.
But you forget that both planes have two trailing vortices, one on the left and one on the right side. So what is left after half of those vortices  cancel each other are two vortices at twice the distance of the vortices of one airplane.
If we neglect dissipation and assume that those two vortices will stay there for some time, they will look just like the vortices left by a single airplane of twice the wingspan and four times the mass of those two which did not collide. However, the downwash between the remaining vortices will look differently and reveal that two separate airplanes were involved in its creation.

Will two counter rotating vortices cancel each other?

Yes

Suppose that an aircraft flies with its wing through an tip vortex of another aircraft which flew in the opposite direction. Suppose that the shed wing tip vortex of both aircraft are exactly the same in strength, only rotating in opposite direction?

Wheather the planes fly in the same direction or opposite directions will have no effect on the vortex directions - remember that the vortices are symmetrical on the lateral axis. Given that the wing produces a positive lift, the left and right vortices will always rotate clockwise and counterclockwise respectively, regardless of the point of view. Look at the following picture, it shows that the vortices rotate in the same direction.

With the aircraft example the circulation is opposite in sign as well. So why is the root vortex a summation of the two blades instead of cancellation?

As seen in the picture, the signs are same, not opposite. The vortices will therefore add up, not cancel out.

A wing that produces negative lift can produce vortices of the opposite direction (left counterclockwise, right clockwise). If a positive lift producing wing was to intercept its wake, then indeed the vortices from the two wings will be opposite and tend to cancel out.
However, there is one more way of producing opposing vortices - without involving a negative lift producing wing. The clockwise vortex of one wing's left-side can cancel out the counterclockwise vortex of another wing's right-side. Indeed the vortices will tend to cancel out, and the two wings will basically act as one larger wing.

I think your picture does not match the intent of the question.  I think the idea was that the flight paths are offset by one wing span, so that the wing tips are aligned and the fuselages are not.

@AnonymousPhysicist  I'm not sure if question indicates that the flight paths are offset by one wingspan...

Yes it is indeed as @AnonymousPhysicist says: I also did the same mistake at the beginning and asked for clarification. So, both left (or right) wing tips overlap. Anyway the question isn't that clear...

For an airplane in level flight, the angular momentum of the aircraft remains zero.  As a result, the air the plane flies through is not gaining angular momentum.
Vortices are not vectors and are not subject to any kind of law of superposition.  In other words, you cannot add them together.  The Navier-Stokes equation is the right way to calculate flow.

Saying "the wing makes a tip vortex" is already an extreme oversimplification.

What do you mean with "the air the plane flies through is not gaining angular momentum"?

No, but the wing tip vortices (or the parcels of air that are typically denoted with tip vortices) DO have angular momentum! Moreover, vortex line codes are derived from the NS stokes equation and the singularities which construct the solution for Laplace equation are these vortex lines (amongst other type of solutions). Therefore, the flow is represented by vortex lines. Basically Kutta, Helmholz, Biot and Savart are the holy fathers here.

@|Windy As mentioned in the other answers, if you consider the vortices from both sides of the same plane, the angular momentum is zero.  The question asked for the "net effect" and I interpret that as including all the air.


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Would an airliner climb higher late in the flight because of the consumed fuel (lighter airplane) to save more fuel?
Some people said airplanes would fly higher in the late period of each flight, because the fuel is consumed, and the airplane is lighter. Higher altitude and less dense air is enough to support the airplane, not to mention, less drag in high altitude can save the fuel further.
This statement sounds reasonable. But would this be practical in the real world?
Because you can always lower your pitch angle, and get less drag and save fuel. In this way, you don't need climb to consume additional fuel. And you don't mess with other traffic.
What would a typical 2-hour domestic flight's altitude change be like?  What about a typical 10-hour international flight?

Related: What is the flight trajectory of a commercial airplane in regards to altitude v/s distance?

What do you mean by "you can always lower your pitch angle, and get less drag and save fuel"? If you pitch down (in level flight at a given airspeed), the AoA decreases and thus lift decreases as well. As that lift is no longer enough to counteract gravity, you will start to descend.

@ymb1 Oh, right, of course. I didn't realize the question doesn't assume trimmed level flight at an optimal cruise airspeed to begin with.

Yes, this is both practical in the real world and very common. Almost all trans-Pacific flights do this (called "step climbs") because they start off much, much heavier than they land. Their initial cruising altitudes need to be relatively low due to their high initial weight, but they're able to climb up into the thinner air once they've burned off hundreds of thousands of pounds of fuel weight and that enables them to fly the remainder of the flight more efficiently.
This is much less common on short 2-hour domestic hops, though. Those typically aren't altitude-limited by their fuel weight in the first place, so they have no need to start off at a lower altitude and then climb later. They just climb straight to a cruising altitude that they'll use all the way to top-of-descent unless they or ATC need them to change altitude for something like weather, turbulence, or traffic. The fuel weight is a much smaller fraction of the overall aircraft takeoff weight on 2-hour domestic hops than it is on 15-hour trans-Pacific long-hauls. This is also why those aircraft can typically land immediately after takeoff if needed and typically don't even have fuel jettison capabilities.
Here's an example of a Tokyo-Narita to New York-JFK flight on a Japan Airlines Boeing 777-300ER earlier this week:

Source: FlightAware
This flight is a bit over 12 hours in duration. It started off with an initial cruise altitude of 32,000 feet. Interestingly, it seems to have initially received a block clearance, taking nearly an hour and 40 minutes to climb from 30,000 to 32,000. It then did a step climb to 33,000 at roughly 3 hours, 34,000 at 4.5 hours, 35,000 at 6.5 hours, and finally 37,000 at 9.5 hours. If you are paying attention as a passenger, you can hear when these step climbs take place, as the engines will get noticeably louder for several minutes or so during the step climbs. Of course, you can also see it on the flight tracker on the in-flight entertainment screens on flights that have those.

Because you can always lower your pitch angle, and get less drag and save fuel. In this way, you don't need climb to consume additional fuel. And you don't mess with other airway traffics.

This is what you'll do to maintain an altitude assignment as you burn off fuel (generally done in very small steps constantly and automatically by the autopilot.) It does reduce fuel burn somewhat, of course, but it doesn't reduce it as much as climbing into the thinner air does.

Yes, the step climb is done to conserve fuel, because the air is thinner there and takes less fuel to push through. (Like the way you'll get an MPG bump driving your car in Wyoming).
They already take the lowest AOA that will result in level flight, and the AOA is high because the airplane is heavy with fuel.  In that condition, pushing the nose down to reduce AOA would result in losing altitude.
it raises the issue "why step-climb, why not start at the higher altitude?"
The answer is they can't.
The even higher AoA required to fly in the thinner air would result in stall.
The first steps are lower because the airplane is too heavy to be able to fly at the higher altitudes.  Only after it loses some weight in fuel is it possible to safely attain the next step upwards.  Then it loses more and takes the next step if able to get clearance.

In horizontal flight, lift L = weight W = $C_L \cdot ½ \rho V^2 \cdot S$ (in incompressible circumstances). If W decreases continuously because of fuel burn, lift will need to decrease accordingly as well. Since wing area S is a fixed value in cruise, lift can be reduced by:

Lower $C_L$ by changing trim;
Lower air density $\rho$ by flying higher;
Lower airspeed V by adjusting throttle;

It turns out that the optimum way to adjust for the reduced weight is by increasing altitude, linearly and continuously, according to the green line in above pic, from this document page 10.. But there are quite a few aeroplanes cruising between continents and they need to be separated in flight corridors, that is why subsonic jetliners perform the cruise climb in steps. Indeed Concorde did not have this issue and could climb continuously, as @ymb1's answer mentions.

The referenced MIT-document is quite an interesting one. From page 13 above, which mentions that density altitude should increase linearly but aircraft fly pressure altitude, which can lead to steps in pressure altitude.


text:
What wind speed should I adhere to (gusts or not)?
I live in the UK, and as such frequently have flight plans cancelled in Microlights (Ikarus C42). Anyway, one of the reasons is it has a low crosswind limit (15 kts). When I look at the weather, I ensure the gusts do not exceed this.
But sometimes the windspeed is under this, say 12 kts, whilst the gusts are much higher, say 30kts. I keep missing flights as I decide not to fly based on gust speeds. Can I just ignore these and focus on the average speed?

Are you a student? I ask because the capabilities of a student are generally less than that of someone with a licence.

@Ben The crosswind limit is set based on the maximum speed a test pilot achieved a reasonably controlled landing at. I don't wish to presume my skills would out-do those of a professional test pilot!

@Cloud “Max demo” doesn’t mean a test pilot couldn’t do more, merely that the mfr didn’t pay them to try anything beyond the design goal.

Gusts are wind bursts of up to 20 seconds, they may be short lived but you should take these into account, so you should use the gust speed, not the steady wind speed. If you are on short final close to your crosswind limit and a gust comes that's higher than your control surfaces can compensate for it doesn't matter how good a pilot you are, the airplane is going to go sideways and there's nothing you can do to prevent it. A brief burst may not sound like a lot but it only takes 2-3 seconds to blow you off the runway.
As painful as it may be you have to take the gust values and your own personal capabilities as a pilot into account. That doesn't necessarily mean that having gusts over the limit is an automatic no-go, there are mitigating factors to consider, especially if it's very close to the limit. Shelter helps, the airfield I fly from has some woods which shelter the landing end of one of the runways, which is the one that's most frequently in use. These woods moderate the wind and gusts from the west, which is prevailing, so I often can fly in higher winds.
The OP is probably well aware of this, but a well rounded answer needs to mention wind direction and calculating the crosswind component. Wind direction is also a consideration, the calculation is V x Sine degrees of crosswind. 30 degrees is 50%, 45 degrees is 70% and 60 degrees is %85. This is hard to calculate in the air, it's easier to say that 45 degrees is 75% and 60 degrees is 100%, and gives a bit of 'fudge factor' for safety. So, if you have a 45 degree crosswind at 20kt gusts you are within limits.

When you say degrees, are you talking about wind heading, or relative to the aircraft?

I mean degrees of crosswind, which is calculated from the difference between the wind angle and runway heading. If your runway is 90 and the winds are from 45 or 135 you have 45 degrees of crosswind.

You've mentioned gust speeds, and you've mentioned crosswind limits of the aircraft (and by extension, your own limitations). But you have not specified the runway direction.
If the gusts are straight down the runway, you may be ok to fly. You will want to increase your final approach speed by (at least) half of the gust speed. If the gusts are directly perpendicular to the runway then you need to check your aircraft limit. Somewhere in between, then you'll want to know what component is a headwind and what is a crosswind and adjust the above accordingly.


text:
How should a pilot respond verbally to being cleared for takeoff?
When ATC says "cleared for takeoff", what's the correct answer from the pilot?

This is how FAA recommends (PDF):

A typical takeoff clearance may state, for example, “(Callsign) 123
  RNAV to MPASS, Runway 26L, Cleared for Takeoff”.
The expected pilot response is, “(Callsign) 123, RNAV to MPASS, Runway
  26L, Cleared for Takeoff”.

It is probably worth adding that this is true of most ATC instructions : http://aviation.stackexchange.com/a/288/3573

Also, if you're taking-off from an intersection of a long runway (many GA aircraft do), one should always try to mention the intersection for safety.

And the opposite too - if the majority of the traffic is taking off from an intersection, and you're taking off full length. Always a good idea to give ATC and others situational awareness.

Same phraseology you're given: cleared for takeoff with your callsign, and if applicable, the runway or instructions ("turn left heading 200" for instance) given in the same transmission. Anything else is nonstandard.
Not on the roll nor on the go nor cleared for departure nor anything else, no matter how cool it sounded when somebody else said it on the radio.

Note, that bad phraseology regarding take-off clearance strongly contributed to the worst aviation disaster in history. That's some good reason to stick to the standard.

@JanHudec where can I read more about that?

@JacobKrall Here

@JanHudec: Great tragedy, lots of lessons.

Quite simply, you readback the runway identifier and the clearance - exactly like you receive it.

or

In case you get other instructions in connection with the takeoff clearance, you need to read those back as well.

Note that you should not read back any wind information given, since it is just that - an information - not a clearance or instruction.
ICAO Document 9432, section 4.5


text:
How much is air mass flow rate of bleed air used for environmental control system in civil aircraft?
I'm looking for a general use-case formula or reference that I can use to calculate the total mass flow rate of bleed air use in the environmental control system of any aircraft. I'm not looking for information on a specific aircraft - simply a general reference that can applies to all civil aircraft.

For what aircraft?

Welcome to the site! Your question is a bit broad and "critical condition" is not so clear. You may edit your post to clarify what you want to know. Overall the ECS packs on usual airliners can be shut-off when necessary, so they will not load engines.

Tnx Mr Beyer.
actually i need any kind of information on this subject. i need to know maximum and minimum mass flow rate . for any civil aircraft.

@ebrahimhajian I apologize but that is way too broad for this site. You need to narrow it down to at least a family of aircraft (777, A320, etc). This differs wildly (I'm sure) based on cabin configuration, aircraft size, and engine selection.

EASA and FAA engine type certificates often identify the total bleed mass flow, as a % of the engine's high pressure compressor exit flow. For example, see section 10, "Maximum permissible air bleed extraction" in this for the CFM56-7B, which shows 7 to 13%. If you assume in a civil aircraft that all the bleed air from the engine if for the ECS, you get an answer. (Or maybe you where actually looking for the total bleed mass flow).
If you can get this data from enough references, you could develop a general guideline.

Compute aircraft internal volume V [m$^3$]
Refresh this volume every 2 minutes
Take an air density at 2,000 m: $\rho$ = 0.94 kg/m$^3$ (cruise altitude)
Compute the mass flow $\dot{m}$ = V * $\rho$ / 120 [kg/s]


text:
Can a plane with 2 sets of shorter wings fly?
I am proposing a plane that has 2 short wings of the same size on either side of the fuselage (two in the front and two in the back). Rather than using the horizontal stabilizer to destroy lift, the wings in the back help create lift and adjust the pitch of the plane.
One problem that I thought of is the wingtip vortex created by the front wing hitting the back wings. I'm guessing this can be resolved by placing the back wings higher than the front wings (from a front view of the fuselage).
Could this work?

By "work" do you mean fly or be more efficient than planes with single wings?

I mean be able to fly. I'd assume that it would be more efficient.

As per my answer,  more and shorter wings -> less efficient.   This is why gliders (most concerned with efficiency) have long wings.   You need to minimise the ratio of end effects to middle of wing efficiency.

The "wing tip" vortex is not generated by the tip, but is a result of the wing downwash that is side-effect of producing lift. Since the wing affects air above and below it to height approximately equal to its span, the aft wing will fly in the downwash of the front wing no matter where you place it.

Note that horizontal stabilizer does not necessarily destroy lift or produce negative lift. If you're looking for unusual designs, think at Scaled composites' aircrafts.

Of course, the "vortex hitting rear wing" could be avoided by placing the two wings above each other, not behind. That's a workable configuration (biplane). And if you don't put the right above each other, you still have some torque to affect the pitch.

Why stop at two?

See also the canard, a relatively small lifting wing ahead of the main wing.

What you are proposing is called a "tandem wing", and it's been done before. A brief search should produce more than enough research papers about the advantages and disadvantages of this particular design.
The near-complete absence of multiple-wing designs in modern aviation should be a hint that the design has issues. Unless you particularly need the advantages of the design more than the drawbacks, there's no particular reason to use it.

What are some disadvantages of the tandem wing?

@EwenW. That would be an excellent question to post to this Stack :)

@EwenW. "what are the advantages" might be a better question

It would be able to "work".   Anything with a strong enough engine pointed in the right direction can fly :)
The question is how well it will work.
A significant issue is that the most of the problems with wings occur where they join to the fuse, and where they end.   It's the middle part of the wing that works optimally.   So if you half the length of your wings and double the number of them you are giving yourself massively increased end effects...
As mentioned in comments, the dynamics of stability are also more challenging when you have two main wings producing lift.

An additional problem is that the usual "main wing and stabilizing tailplane" configuration (where the tailplane produces negative lift in level flight) will naturally tend back towards level flight after a minor deviation in pitch, whereas having a fore and an aft wing that both lift would need additional arrangements for pitch stability.

@HenningMakholm: Not quite. Just let the rear wing produce less lift per unit of area than the front wing, and the configuration is stable in the same way as a conventional configuration. If you desire a longer explanation, ask a new question.

@PeterKämpf broadly speaking you just listed "additional arrangements"

You also want the front wing to stall before the rear wing. Otherwise the stall will be violently nose up and difficult to recover from. I think this is essential if you want to call the configuration flyable. That goes together with having the rear wing more lightly loaded as @Peter Kämpf says.

@Wirewrap from what I've looked at the tandem wings make it very hard to stall.

That will depend entirely on the relative angles and dynamics of the wings.   You could set it up so that the rear wing stalls first, which would be deadly...

First the answer to your question: Yes, it can.
The probably most popular tandem wing was the French "Sky louse" (Pou du Ciel), a design from the 1930's that was built in countless variations.

Mignet HM 14 Pou du Ciel (picture source). The initial type flew with a motorcycle engine of 17 HP. OK, the wings are not of equal size, but nearly. Close enough to count.
But the type had issues due to the small distance between both wings. Please read this answer for details.
And now please do me (and, hopefully, yourself) a favor and see the "lift destroying" elevator with different eyes. This lower lift (or even downforce) is only the consequence of static longitudinal stability, and both can easily be eliminated by shifting the center of gravity back to the neutral point of a configuration. But if you desire to have some natural stability, you will need to accept "lift destruction" regardless of the configuration. To paraphrase a popular song: Birds do it, bees do it …
… even educated flying wings do it, I might continue. Canards, tandem wings, they all will use the rear part of their lifting surfaces less if they want to achieve static stability. And the most efficient way of achieving it is by using the smallest surface of two for this lower lift. This has the additional benefit that the "rear wing" will be entirely within the downwash field of the forward wing, so it will see less angle of attack variation than the wing and be in a uniform flow field.

There are many good reasons for a tandem wing, and a few bad ones.
A few responses mentioned influence factors from the forwards wing, that is not bad if it is designed in.  With any design, including flying wings, the "tail" is built into the rear (or rear portion) of the wing.  So a flying wing or canard will have more wing area than a 'conventional' plane of the same performance.  For efficiency, you want long slender wings.  If you made two long slender wings in tandem, you wind up making the chord (wing width) shorter, which put you into a lower Reynolds number which means higher drag, and your airfoil thickness is less, which means your spar height is less, which could mean a heavier wing.
On the bright side, your fuselage becomes lighter (weight being distributed more evenly) and you can build in stall resistance.  For some aircraft, like the Ligeti Stratos, you can join the wings to add strength (like a biplane).  Shorter wings means a higher roll rate, smaller wings could means better control in turbulance.  Tandem wings also offer control coupling to make things like translation without rotation possible.
Most of all, I just think it goods good.

Another point that was not brought up yet is the rotational center of the airframe - specifically pitch and yaw would require much more effort to rotate. When you have single set of wings in the center of gravity, rotating the airframe takes less effort compared to when you have 2 sets of wings and center of gravity in the middle.


text:
What are the "lumps" in the A380 flight-deck?
In this photograph of the Airbus A830 flight-deck there are two large "lumps" sticking up and out from the centre console, next to the engine thrust levers and flight-computer keypads:

I think they're just meant to be wrist-support for when pressing the keypad keys, but it seems odd to single-out that part of the flight-controls in particular for wrist-support. The lumps also seem rather tall to be used comfortably for extended periods (and I assume that's what the pull-out keyboards in the trays are for, so why bother having secondary keyboards down there in the first place?) - and it also hinders access to the 'inner' keys closer to the engine controls.

They are also great hand holds for getting into/out of the seat!!

They are a different design of computer mouse:

As far as I can tell, the front disk is a track pad and the model in your picture has the mouse buttons on the front instead of the sides:

Can you provide a link to the manufacturer's website or other current source?

I recently saw a video about the last flight of Jürgen "J.R." Raps who was chief pilot (I don't know the proper term) at Lufthansa. He explained some of the equipement of an A380, one of which was the mouse. "The first thing you see are the numerous screens and two “palm stones” that resemble ostrich eggs – which turn out to be computer mice especially designed for pilots. They make it possible to move the cursor properly even when there is turbulence." (http://pilotseye.tv/en/route/a380-san-francisco-the-final-flights-of-jr-juergen-raps/)

Here Jürgen Raps explains the controls of an A380. Jump to 3:40 to see the mouse in action.

The picture is from this company's website: https://www.thalesgroup.com/en

I wonder if I can get a mouse like this for my desktop computer!

How do the mice work? I can't tell from the photos if they're trackballs or capacitative trackpads - or something else.

I think they are trackpads in this case. However, I have been in a Pilatus PC-12 that had a trackball design.


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Are thrust levers synchronized by default when pushed/pulled?
When I watched this video about the last flight of Juergen "J.R." Raps from SFO to FRA in an A380 (before his retirement) I noticed he lines up or synchronizes the four thrust levers as shown between 08:05 and 08:20 (while taxiing, if that matters).

I used to think the levers are synchronized by default (same precentage of thrust on each engine) and it isn't crucial to have the levers in a straight line as long as the difference between their positions is below a reasonable amount. I further thought that in case the thrust needs to be distributed asymmetrically for whatever reason (say 70% board and 60% starboard), one would need to press some button on the levers to release some sort of lock and move them separately instead of simultaneously.
I personally would find it hard to move four levers simultaneously at the same angle, let alone people with small hands who may only grab three of them at once.
So how does it work? Or is the pilot's behaviour just a personal habit?

On a non-FADEC engine (older airplanes - not this one) where the thrust levers are connected to the engine fuel control units with a cable circuit or a teleflex cable, you have to manually adjust each lever to wherever it has to go to achieve a given N1 (fan speed), and this can result in small variations in position of each lever when all engines are at the same power setting.  On takeoff on a non-FADEC, you move the thrust levers until power is reasonably close to take off thrust, call "set thrust", and the non-flying pilot tweaks each lever to synchronize all 2 or 4 N1 values to the takeoff bug setting on the N1 indicators.  With mechanical controls, the thrust levers may or may not be perfectly aligned when the engines themselves are in synch (there are limits to the misalignment allowed).
On a FADEC aircraft like the 380 in the video, the engines are managed by computers, and the thrust levers are just electrical input devices.  At the bottom of each lever there are Rotary Variable Displacement Transducers (RVDTs), that tell each engine's FADEC computer where the thrust lever is.  The FADEC will manage the N1 and if all 4 levers are at the same position, each FADEC will achieve the same N1 (within the tolerance of the RVDT's signal).
For takeoff, to make it easy, there is a detent for each lever at the Takeoff Go-Around setting (TOGA) that you can feel as you move them up (a little roller drops into a depression on a cam profile on the lever's arc of rotation).  There is another detent for climb power as well (CL) to avoid having to fuss with lever settings during the departure. When in the TOGA or CLIMB detent, the FADEC computer takes care of setting its engine's computed takeoff or climb power automatically.  So on a FADEC engine, when you take off, you just push the levers until you feel each one click into its TOGA detent, call "set thrust", and the non flying pilot just looks at the N1 indications to confirm they are all at TOGA thrust, without having to tweak the thrust levers (on the RJ I used to touch the levers below the handles as the PF was pushing them up with my finger and thumb to feel for myself that the levers clicked into the detents, although it wasn't really necessary).
At settings other than TOGA or CLIMB thrust however, they tend to work like mechanical thrust levers, where if one lever is at a different position it will have a different N1 value, so at intermediate power settings or on approach you have to individually align the levers, so you end up moving them as together as best you can and fine tuning as required.
What the pilot in the video is doing is synching the levers for taxi because when he pushed the levers up he didn't push them evenly and it looks like the outer ones were a little farther ahead than the inner ones.  Not that big a deal for taxi; he's just being a perfectionist.

I didn't get into the autothrottle part.  In that case there are servos that drive the levers and the computer does everything.  You just have to get them started.

No, the thrust levers are not automatically synchronized. You could set one engine to 100% and the other to full reverse thrust, just by moving the levers.
Having said that, there are some airplanes that kind of do what you're talking about. For instance, most Airbus designs have detents in the throttle position which basically tell the computer to take over adjusting engine thrust. In the video you linked, at 13:57, the pilot just shoves the throttles into the "Takeoff/Go around" detent, trusting the computers to synchronize the thrust of each engine. Also, a lot of multi-engine propeller-driven airplanes have a synchronization system that keeps both engines at the same RPM, because having two propellers at slightly different speeds creates this pulsing drone sound that a lot of people find annoying. But each of these has to be specifically selected by the pilot, there's no automatic synchronization between engines.


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What is that instrument with two knobs and what looks like range rings?
I just flew a Piper Warrior today and I got confused with the instrument shown below, it's the one above the CDI and to the right of the altimeter.

First search result on Google image search: https://flyingcolorsairparts.com/20719


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What is the piece that covers the top part of tires?
I would like to know the name and the usefulness of this piece that covers the top of the tire of some planes, such as some Cessnas.

Related: What is “spatted undercarriage”?

In general, a "fairing" is something you add on a "thing" so as to make it smoother to go through the air.  You could have a fairing for an antenna, a fairing for some sort of instrument, and so on. I suppose the things in your picture are called "wheel fairings" !

Those are called speed fairings.
They make the shape over the tire more aerodynamic and reduce drag, thus increasing airspeed and fuel efficiency.
This is a page in a Cessna 172R information manual that comments on the differences in performance when they are installed.

Alternate name: "Wheel Pants."

I have never heard/seen them being called "speed fairings" before.

In general, any body part of a vehicle whose sole purpose is to reduce drag by streamlining pointy bits is called a "fairing".

In the UK they are often called 'wheel spats'

@DaveGremlin Wiki says, even "trousers" is used if they also enclose the landing gear.

My first instinct was to say "wheel fairings", since they are fairings around the wheels. Every fairing is a speed fairing (echoing a comment above). But I suppose the only optional speed fairings on a Cessna are the ones on the wheels, and "speed fairings" probably markets better.

"Wheel pants" is a common term for these wheel fairings, at least in US usage, which serve to reduce drag.  They have also been called "spats", though that term has traditionally been reserved for something much larger, at least in US usage.
Related --
How can drag induced by the landing gear be reduced?? -- see all answers
What is "spatted undercarriage"? -- see all answers
https://www.eaa.org/eaa/aircraft-building/building-your-aircraft/while-youre-building/building-articles/landing-gear-wheels-and-brakes/how-to-install-wheel-pants
https://en.wikipedia.org/wiki/Aircraft_fairing
https://en.wikipedia.org/wiki/SOCATA_TB_family
https://en.wikipedia.org/wiki/Northrop_Gamma

Some people call them gear socks, but yeah, if you had a problem with them and needed to notify ATC, you would probably call them wheel fairings.


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What does "Boeing" mean?
We know about the existence of Boeing planes like the 737, 747, and other planes. But where did the name "Boeing" come from?

It's the noise the plane makes when it bounces, obviously ;)

@Baldrickk bounces? What an untechnical term. You mean, when it elastically lithobrakes.

In light of recent events I submit: Because Obviously Everyone Is Not Going

It used to be an everyday slang term in Boston and San Francisco. It means "We need better streetcars".

Its the noise the stock price of BA makes.

It's named for one of its founders William E. Boeing which is the American spelling of his father's German surname "Böing".
To answer the question directly: it does not mean anything in particular.

Well technically Boeing was co-founded by both William Boeing and Conrad Westervelt.  It’s first name was B&W Seaplanes

I think the "Westervelt 747" has a much nicer ring to it...  Or, "Watch out for that Westervelt B-52 coming to bomb you back into the stone age!"

Funfact: in German, the word "Bö" or "Windbö" means gust or squall. It's a bit of nomen est omen but that is by pure chance.

It's not an American spelling so much as a Middle High German spelling.

According to this German newspaper article, it means "son of Boie". The origins of Boie are unknown :)

@PerlDuck I think there's a word for this, 'Nominative Determinism' https://en.wikipedia.org/wiki/Nominative_determinism

the name "Boing aircraft" might well be appropriate for the company mow :)

@A.Rowan "The idea that people are drawn to professions that fit their name was suggested by psychologist Carl Jung, citing as an example Sigmund Freud who studied pleasure and whose surname means "joy"." - sounds like Jung was not studying boys at the time, thus discrediting his own theory.

If it was Böing, that'd mean that basically everyone nowadays pronounces the company name wrong.. ;-)

@WooShell That is already the case though, since "oe" replaces "ö" in German but is pronounced the same.

@EricTowers in addition, it's also the official modern German spelling in contexts where the ö character is unavailable.

@user253751 Jung was silly in that, and clearly didn't know the history of surnames in Europe. Originally people took their profession as a surname more often than not, as they were known as "baker" or "plowman" anyway. As quite often children took after their parents' professions, the impression might exist that people chose their profession based on their name rather than the other way around.

("Jung" is German for "boy")

@user253751 No it isn't.  "jung" is an adjective meaning "young".  "Boy" is "der Junge" (note the trailing 'e')

@MartinBonnersupportsMonica Depending on the local dialect, "Junge" can be instead "Jung" or "Jong". Maybe user253751 stems from a region with that kind of dialect. But you are correct in that "Junge" is correct German.


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How are air routes created?
How are air routes created? Does it need ‘approval’ from another country? Nepal seems to have built several new international airports without air routes (or having to use the old one to TIA) and it’s being said India has to approve them and it apparently hasn’t yet.
Example airports : Gautam Buddha international Airport, Pokhara International Airport.
Reference: https://myrepublica.nagariknetwork.com/amp/nepal-s-2nd-int-l-airport-ready-for-opening-amid-air-route-issues-with-india/news.html.twig

Related: https://en.wikipedia.org/wiki/Freedoms_of_the_air Note that "freedoms" are not universal, each country can grant (or not grant) whatever freedoms they want to other countries, often via bilateral agreements (you allow my aircraft to do this in your country, I will allow yours to do the same in mine, usually with conditions attached like number of flights, fees, etc.)

thank you, the link clears a lot of basics @jcaron.

Within its borders a country can create any system of routes it likes, without reference to any other country.
However, a route is only useful if it goes somewhere, so neighbouring countries must agree routes between them so that a contiguous route crosses the border.
The gist of the article you refer to is that the new Gautam Buddha airport will require access to Indian airspace for some operations, but that India is dragging its heels in granting such access.
A communiqué issued in 2014 suggested that access would be granted, but it seems it hasn't happened yet, for whatever reasons India has.

Makes sense @catchAsCatchCan. Looks like because the country is so small, part of the flight descent and ascent occurs in Indian airspace during arrival and departure respectively and thus an "agreement" is required. India "seems" to allow flights in and out of Nepal at or above 21000 ft without issues. https://www.airlimousines.in/index.php/aircrafts-charter-private-jets-faqs/aircraft-aviation-news/item/460-nepal-india-air-route-talks-put-off-until-december.html


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Pilot NOT confirming checklist in Sully (2016). A script error?
In Sully (2016) there's a short moment when pilot flying (captain) confirms something exactly opposite to the checklist item just read by pilot not flying (first officer):

FO: Emergency electrical power, emergency generator not online.
CP: Online.

Source:

Am I missing something obvious or this is a script error? Are there any situations when any checlist item must be negated?

I was thinking about it, then I realized, I've never watched the movie. Just little clips.

Whilst this may not be an error, there is script difference just before this after the mayday call when ATC asks Capt Sullenberger which engines he lost. In reality the question was asked by LaGuardia tower to the New York Departure controller when then mayday was passed on. The New York controller got the both engines out first time.

@JohnK It seems like it's going to be a dry retelling of events of which we already know the outcome, but it's actually quite engaging and well-made.

It's Hollywood, not a documentary. (But it is a good film).

The CVR voice transcript shows that the movie is accurate.
Cactus 1549 CVR voice transcript

15:28:14 HOT-2 “emergency electrical power... emergency generator not online.”
15:28:18 CAM [sound similar to electrical noise from engine igniters ends]
15:28:19 HOT-1 “(it's/is) online.”

The Engine Dual Failure Checklist actually reads:
“EMER ELEC PWR (if EMER GEN not on-line)……………... MAN ON”
The FO misread the checklist and said “emergency generator not online” when he should have said “if emergency generator not online”. The emergency generator was actually online, and the Captain corrected him by saying “(it's/is) online”.
Page 55 of the NTSB Report confirms this:

“Regarding step e, the pilots stated that they determined that electrical power was established and, therefore, that the RAT did not need to be manually deployed. Further, immediately after the loss of engine thrust, the captain started the APU.”

The only error in the movie is they missed the “(it's/is)” part.

I am not a pilot. Is it required/preferred that a checklist is read out verbatim? I would imagine so, but at the same time I would imagine pilots are extremely familiar with this checklist and in this case a bit rushed. The FO was paraphrasing the text as a prompt to both for the action to be taken? And the captain is affirming that it is online.

Checklists are designed to be read verbatim, but in practice they are often misread or paraphrased. Even an experienced pilot can easily make an error, especially in an emergency situation. In this case the EMER GEN was designed to automatically come on as the RAT was deployed. The “MAN ON” response would only be made if there was a failure in the auto system.
It is possible that FO Skiles was paraphrasing and would have continued the with checklist, and correctly pointed out that the EMER GEN was online, and didn’t need to be manually turned on.

Capt Sullenberger was a very experienced pilot, with a training background, and I suspect he spotted the error without even having to look up, and simply stepped in with the correct response instinctively to both save time, and correct any possible error by the FO reading the checklist.

I remember reading or listening to an interview SOMEWHERE where Sully said the first thing he did, upon realizing what happened, was start the APU. That was why it was online when the checklist item was read.

@RetiredATC The RAT powers the Emergency Generator via the Blue Hydraulic system. The APU powers a separate APU generator.

@RetiredATC - that's in the NTSB report. It's not every day that you see in an NTSB report that a pilot didn't follow directions... and that it improved the outcome.

"Although the flight crew was only able to complete about one-third of the Engine Dual
Failure checklist, immediately after the bird strike, the captain did accomplish one critical item that the flight crew did not reach in the checklist: starting the APU. Starting the APU early in the accident sequence proved to be critical because it improved the outcome of the ditching by ensuring that electrical power was available to the airplane. Further, if the captain had not started the APU, the airplane would not have remained in normal law mode...."

"...This critical step would not have been completed if the flight crew had simply followed the order of the items in the checklist. The NTSB concludes that, despite being unable to complete the Engine Dual Failure checklist, the captain started the APU, which improved the outcome of the ditching by ensuring that a primary source of electrical power was available to the airplane and that the airplane remained in normal law and maintained the flight envelope protections, one of which protects against a stall."


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Is it suspicious or odd to stand by the gate of a GA airport watching the planes?
Would it be likely to be seen as odd or suspicious for someone who is not a pilot to stand outside the fence around a GA airport in the US watching the aircraft?

If you're lucky sometime will show you around and/or let you hang out for a flight.

@tedder42  I agree, that's the way it works in the UK, dunno about the US.  But I just remembered that a bunch of plane spotters were charged with spying in Greece a while back at a military base:  http://news.bbc.co.uk/onthisday/hi/dates/stories/november/12/newsid_2518000/2518385.stm   Also, I was prevented from taking photos at a small (commercal) regional airport in India a couple of years ago...

Not related to your question about the US, but a lot of countries have shared commercial and military airports and that kind of activity would probably draw unwanted attention from the authorities.

Binoculars may make you look less suspicious.

Jus watch on Big Jet TV on youtube https://www.youtube.com/c/BigJetTV?app=desktop https://www.bbc.co.uk/news/entertainment-arts-60434909

@DaveGremlin Those idiots were actually on the premises of the airbase, and trying to hide from security. Never a good idea.

"Suspicious" is in the eye of the beholder.

@jwenting Also, there's a BIG difference between a military base and a GA airport.  You'd get a similar reaction hanging around at any military base, regardless of whether it had an airstrip.

@DarrelHoffman true, but usually if you just hang out outside the airbase perimeter you're left alone, you'll at most be watched from a patrol car or other security staff. These guys in Greece had breached the airbase perimeter and were walking on the grounds, knowingly, then fled when approached by guards.

I mainly fly in the UK and Europe, however I trained in the US and have flown there a few times since. I have probably 50 hours in total in the US.
I can tell you that almost without fail, every airfield I have ever flown from anywhere in the world has always had some level of plane spotting activity. It is a hugely popular pastime with both photographers and people who just like watching.
I don't think there's anything particularly suspicious about it, so long as you're not trespassing, not touching anything you shouldn't be, and not causing any interferrence.

It depends on how you go about it.
Don't wear all black or camo with a face mask.  Don't sneak around in the grass, or run from building to building seeking cover.
Plane spotting is a somewhat common hobby -- you might check out some plane spotting forums for tips on not getting hassled.  You might think having a camera with a lens as long as your arm would be suspicious...  yeah, it is still suspicious.
Many GA airports have a restaurant or a designated viewing area.  My local small airport (KSBP) has both of these.  It also has a picnic table along a back road near mid field at a particularly good vantage point.  At the other end of the field, there is a wide spot in the road directly under the ILS approach path that people frequently use.
You should be able to find some contact information for an airport -- either a local airport authority, an FBO, or some other on airport business.  Give them a call, they can probably tell you the best vantage point.
Of course, the best place to watch planes on approach at my airport is from one of the wineries under the approach path.  Tolosa has a lovely garden with some seating areas.  You can take your tasting outside and watch airplanes land from a couch.  I realize that is not a solution that works for everybody, but it is good if you can get it.

There is no restaurant or anything like that at this airport. The only "businesses" are the local office for an air ambulance company, and the flying club's self service fuel station, which is behind the gate. There are parking areas on both sides of the fence; it's probably fine to park in the outside parking area and watch from there? And maybe walk around in the parking area? Any photography would be with a phone.

Any of those sound great.  Realize that people who work at an airport are likely airplane people -- they love watching airplanes too.  Call up the flying club and they'll tell you the best places to watch from.  I was joking about the camera, plane spotters have a stereotype of wearing a camera vest with lots of equipment and several huge lenses.  Use whatever you'e got.  Have a great time!

And if you just came from the hardware store where you bought a 4' section of black 3" PVC pipe, leave it in the car.  Don't watch planes with it across one shoulder.

Agreed. Asking the people in the flying club with both likely get you good advice for the specific airport you have in mind and also will make your intentions known to them. As long as the airport involved isn't something like Edwards AFB or Groom Lake, the local pilots are generally going to be understanding and friendly to people who stand around and watch planes because they probably do that, too. :)

@RobMcDonald I've got a 400mm zoom lens on my camera... not quite as long as my arm but it does stick out a bit at the tightest zoom.  I'm not near an airport though, I take pictures of plane and helicopters mostly that fly over my house.  It's always fun the take pictures of police helicopters because I can see which way their camera is pointing when I zoom in on the resulting picture.

@MichaelHall Yes yes, that gets filed in the same category as "Don't put candles wrapped around an alarm clock or a bowling ball with a rope sticking out of it in your luggage"

This might sound silly, but I have a fluorescent yellow vest. Should I wear it?

@Someone That would be sensible if you're going somewhere that ground traffic will be passing near you, e.g., you might have to cross roads to get to a good plane spotting location and you might be distracted by a plane when you should be looking out for cars.

@Someone Yes, and carry a clipboard with you too... that should get you anywhere.

@Michael I don't get it; why the clipboard? Do FAA inspectors wear yellow vests and carry clipboards or something?

@Someone The clipboard thing is a joke: if you look official with a high-vis vest and a clipboard, then people will assume you are allowed to be there. I suspect you would need a hard hat and new-looking steel toe-cap boots to complete the outfit.

Clipboard used to be the standard go-anywhere solution. For better or worse, it doesn't work as well any more. But I have a very brightly colored jacket that I got for free (another story...) and I wear just because I have it (comfortable, warm enough for spring/fall weather). Wearing that jacket I have been mistaken for road/construction workers...

@manassehkatz-Moving2Codidact Modern replacement for a clipboard would just be an iPad or equivalent tablet.  Preferably one with a ruggedized protective casing around it so it looks like you bring this to worksites on a regular basis.

@Someone, A yellow vest might not be the best idea, it could be perceived as trying to impersonate authorized personnel if that's what they wear.

Being from Arizona, the only major airports I know of are Gateway and Sky Harbor. Both of which have plenty of plane watchers. As a previous answer has stated, don't touch anything, don't be sneaking around, and don't suddenly dart away as soon as you are seen, as people may think you're planning to hijack a plane, escape security, or commit a crime. Try not to wear all back or camouflage clothing, especially if you do happen to be sneaking around, darting off, and touching stuff, as that will make you even more suspicious. Other than that, most people aren't suspicious of plane watchers.
EDIT: As for GA airports: the only one I know of is in Iowa and I barely remember going there (I was about 2 or 3 at the time) but according to my father, the fences surrounding the airport were lined with plane watchers so I'd think from that information it's totally fine.

Hi, Welcome to Aviation.SE! Not sure if you're familiar with the term or not, but the "GA" that the question is referring to means "general aviation," which is more-or-less FAA-speak for "not commercial or military." When people talk about "GA airports," they normally mean small regional airports used mostly or exclusively by private individuals or companies for their light aircraft or business jets, so generally airports that are much, much smaller than Sky Harbor with no passenger terminal. Generally something more like the Sedona airport.

and oh, stay outside the airfield boundaries unless the spot is a clearly marked dedicated spotting area, and don't even touch the perimeter fence.